Because they store a stream of digitized data into a memory, digital oscilloscopes have powerful features concerning how they may be triggered. For example, unlike an analog 'scope that must be triggered before it commences a trace, the digital 'scope can be instructed to display the waveform(s) preceding a trigger event (so called "negative time"). Indeed, the memory of a digital oscilloscope is often large enough that regardless of the selected time scale (unit of time per horizontal division, and corresponding to the "sweep speed" of an analog 'scope) there is more stored waveform data than can be displayed on the screen, and the user has been given the ability to specify (within limits determined in part by the size of the memory) how the display is to be oriented relative to the trigger event. In one manufacturer's series of digital 'scopes this process involves: (1) Specifying an arbitrary separation in time (delay) between the trigger event and a location that may be called the "trigger reference point", or TRP; and, (2) Specifying a location in the display (left end, middle or right end) where the TRP is to appear within the visible trace. For example, when the 'scope first turns on it may default to: (1) A separation (delay) of zero; and, (2) Placement of the TRP in the middle of the display. This means that the left half of the trace occurred before the trigger event, the trigger event corresponds to the middle of the trace, and that the right half of the trace occurred subsequent to the trigger. This is a flexible architecture that allows many desirable operations. For example, data acquisition can be halted (the 'scope "stopped") and the display panned and zoomed (within limits imposed by memory) by changing the time scale and the delay between the trigger event and the TRP, all after the fact.
Another feature often present in such digital 'scopes is the ability to examine the digitized data stored in memory and automatically measure selected parameters of interest, such as frequency, voltage excursions, and rise times. Very often the parameter to be measured is not something that pertains to the entire waveform stored in the memory (as does, say, RMS voltage) but instead pertains to an individual cycle of a signal, of which there may be several cycles in the digitized data in memory. An example is the measurement of rise time. Some rule must exist to select which particular cycle it is whose rise time will be automatically measured. (Non-automatic measurement is also possible, and involves a manually performed indication of where the measurement is to occur.) A conventional practice in a digital oscilloscope that is "running" (i.e., is adding digitized data to acquisition records in memory, which allows the displayed trace to change and reflect the most recent waveform shape) is to measure the earliest one that is visible in the display. It is also conventional to somehow indicate where on the trace the parameter was measured, say, either by intensification or by bracketing that region with cursors. That is all well and good, but suppose that the 'scope is then "stopped" (i.e., data acquisition halted and the content of memory frozen), and some adjustment made about what portion of the captured waveform is to be displayed as the trace. Unexpected or aggravating circumstances may ensue.
For example, suppose that rise times were being measured automatically, that more than one rising edge is present in the displayed trace, that the left-most visible thereof is measured, and that the TRP is in the center. Let there be an unmeasured rising edge that is fairly close to the TRP. Now the operator stops the 'scope and alters the delay and/or time scale (think: zooms in or out) to view the measured rising edge in a context that places it in the middle of preceding and subsequent rising edges, so that he may better appreciate the circumstances. Since the rising edge of interest is no longer the first within the visible display, the measurement is now performed on a different rising edge (because it is now the first visible rising edge). This change in instance of the parameter measured is made without warning, and might easily be overlooked by an unwary operator. To be sure, he can use conventional means to force the measurement to be on the rising edge of interest by overriding the automatic selection process with manually positioned cursors. But this is both an aggravation and a potential source of error, since he might not position the cursors on the selected feature in the same way as the automatic algorithm does. For instance, he might not position the cursors on the same start and stop percentages of excursion for rise time that the 'scope would select.
It would be desirable if a stopped digital 'scope allowed the operator to zoom and pan to alter the display of a captured waveform while minimizing incidental changes that alter which instance of an automatically measured parameter is the measured instance.
It would be especially desirable if the minimization of incidental alterations of the automatically selected measured instance were also operative even when the digital 'scope were running, as this would allow the operator to alter the display settings and still see the same automatically selected measurement as before, and it would reduce the chance that the operator would become confused about which instance is which during extensive manipulation of the display controls.
It would also be desirable if the operator were informed by the digital 'scope if there were a change in automatically selected instance.